Film with metal foil

ABSTRACT

In a film with metal foil of the present invention, a metal foil is stuck to the surface of a resin film via an adhesive layer. The adhesive is formed by crosslinking an acrylic polymer obtained by the copolymerization of a (meth)acrylic acid ester with a carboxyl group-containing radically polymerizable monomer, with a polyfunctional compound having a functional group reactive with the carboxyl group. The film with metal foil is very useful for producing a multi-layer wiring board by the so-called transfer method. By using this film, there can be produced a multi-layer wiring board having a fine and highly dense wiring/circuit layer and having a very excellently flat surface.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a film with metal foil. Morespecifically, the invention relates to a film with metal foil useful forproducing a circuit sheet which is used in the production of amulti-layer wiring board such as a package for containing asemiconductor element and to a film with metal foil useful for producingthe multi-layer wiring board from the circuit sheet.

[0003] 2. Description of the Prior Art

[0004] It is a modern trend toward producing electronic devices in eversmall sizes. In these circumstances, the development of portable dataterminals and the widespread use of a so-called mobile computing forexecuting the operation by bringing a computer, have further urged thedevices to be realized in smaller sizes and the multi-layer wiring boardcontained in the electronic devices to be fabricated in smaller sizesand in smaller thicknesses yet forming more fine circuitry.

[0005] Further, electronic equipment that must operate at high speedshave been widely used as represented by communications equipment. Inorder to cope with such electronic equipment, it has been urged toprovide a multi-layer printed wiring board adapted to high-speedoperation. The high-speed operation involves a variety of requirementssuch as a correct switching operation for the signals of a highfrequency. To execute the high-speed operation, it becomes necessary toshorten the length of the wirings, to decrease the width of the wirings,to decrease the gap among the wirings, and to shorten the time requiredfor the propagation of electric signals. That is, the multi-layer wiringboard must be realized in a small size, in a decreased thickness andforming a fine circuitry (highly dense circuitry) even from thestandpoint of coping with a high-speed operation.

[0006] A build-up method has been known for producing a multi-layerwiring board that satisfies the above-mentioned requirements.

[0007] According to the build-up method, first, a wiring/circuit layeris formed on the surface of an insulating board made of a glass-epoxycomposite material, and through-hole conductors are formed so as to beelectrically joined to the wiring/circuit layer on the surface, therebyto fabricate a core board.

[0008] Next, a photosensitive resin is applied onto the surface of thecore board to form an insulating layer, which is then exposed to lightand is developed to form via-holes in the insulating layer.

[0009] A layer such as of copper or the like is plated on the wholesurface of the insulating layer inclusive of the surfaces of thevia-holes. Further, a photosensitive resist is applied onto the surfaceof the plated layer followed by exposure to light, developing, etchingand removal of resist to form the wiring/circuit layer.

[0010] Next, as required, the insulating layer is formed and thewiring/circuit layer is formed by using the resist repetitively in orderto obtain a wiring board of a multi-layer structure having a pluralityof circuit boards laminated on the core board.

[0011] In recent years, further, there has been developed a build-upmethod in which a copper foil onto which an uncured thermosetting resinis applied is laminated on a core board, instead of laminating aninsulating layer on the core board using a photosensitive resin.

[0012] That is, the copper foil is stuck onto the surface of the coreboard by the hot press or the like method with the uncured thermosettingresin layer sandwiched therebetween, followed by heating to cure thethermosetting resin thereby to form an insulating layer having a copperfoil on the surface. Then, via-holes are formed in the copper foil andin the insulting layer by using a carbonic acid gas laser or the likelaser. Then, in the same manner as the above-mentioned method, a platedlayer is formed, resist is applied, exposure to light is effected,developing is effected, etching is effected and the resist is removed toform a wiring/circuit layer. Then, as required, the above-mentionedsteps are repeated to obtain a wiring board having a multi-layerstructure in which a plurality of circuit boards are laminated on thecore board.

[0013] However, the following problems are involved when it is attemptedto produce the multi-layer wiring board relying upon the above-mentionedbuild-up method.

[0014] A first problem is that the resin constituting an insulatinglayer laminated on the core board loses characteristics. That is,according to the above build-up method, a photosensitive epoxy resin isgenerally used for forming an insulating layer. The epoxy resin,however, has a low glass transition point and further has photosensitiveproperty. When the multi-layer wiring board obtained by this method isleft to stand, therefore, the coefficient of water absorption increases.When left to stand under high-temperature and high-humidity conditions,in particular, the multi-layer wiring board loses insulating propertyand, hence, loses reliability of the circuit.

[0015] A second problem is that the circuit is not intimately adhered.According to the above-mentioned build-up method in which thewiring/circuit layer is formed on the insulting layer by the platingmethod, in particular, the adhesive strength is small between thewiring/circuit layer and the insulating layer. When, for example, theobtained multi-layer wiring board is heated by solder reflow or thelike, the wiring/circuit layer is peeled off or swells.

[0016] A third problem is that the obtained multi-layer wiring boardlacks surface smoothness. For example, according to the former build-upmethod for forming the insulating layer by applying the photosensitiveresin, the photosensitive resin is in a liquid form. Accordingly,ruggedness in the surface of the core board is reflected up to thesurface of the multi-layer wiring board that is built up to formruggedness. It is expected that directly connecting a silicon chip suchas flip chip on the surface of the multi-layer wiring board will becomea main stream of mounting the silicon chips in the future. However, theflip chip mounting requires a high degree of flatness on the surface ofthe board, and the board having a rugged surface as described above doesnot permit the silicon chips to be mounted.

[0017] The latter build-up method that uses a copper foil onto which ahalf-cured thermosetting resin is applied, is superior to the formerbuild-up method that involves the above-mentioned first and secondproblems. However, the wiring/circuit layer formed on the surface of theinsulating layer is protruding beyond the surface of the insulatinglayer still involving the third problem which is concerned with thesurface flatness.

[0018] Besides, according to the latter build-up method, thewiring/circuit layer is formed by plating copper on the surface of thecopper foil. Therefore, the wiring/circuit layer is thick making itdifficult to form a highly dense and fine wiring/circuit layer.Accordingly, improvements are required such as decreasing the thicknessof the copper foil by half-etching.

SUMMARY OF THE INVENTION

[0019] It is therefore an object of the present invention to provide afilm with metal foil useful for the manufacture of a multi-layer wiringboard having fine and highly dense wiring/circuit layers and featuringexcellent surface flatness, overcoming various problems inherent in thebuild-up method.

[0020] Another object of the present invention is to provide a method ofproducing a circuit sheet for constituting the layers of the multi-layerwiring board and a method of producing the multi-layer wiring boardusing the circuit sheets.

[0021] According to the present invention, there is provided a film withmetal foil in which a metal foil is stuck to one surface of a resin filmvia an adhesive layer; wherein

[0022] said adhesive layer is formed of an adhesive obtained bycrosslinking an acrylic polymer obtained by the copolymerization of a(meth)acrylic acid ester with a carboxyl group-containing radicallypolymerizable monomer, with a polyfunctional compound having afunctional group reactive with the carboxyl group; and

[0023] said adhesive has a storage modulus of shearing elasticity G′ ofnot smaller than 2×10⁵ dynes/cm² (2×10⁴ Pa) and a loss tangent tan δ ofnot smaller than 0.1 at 40° C., and a storage modulus of shearingelasticity G′ of not smaller than 5×10⁵ dynes/cm² (5×10⁴ Pa) and a losstangent tan δ of not larger than 0.04 at 120° C.

[0024] According to the present invention, there is further provided amethod of producing a circuit sheet for a multi-layer wiring boardcomprising:

[0025] working a metal foil of a film with metal foil into a circuitpattern by a resist method thereby to prepare a transfer sheet having awiring/circuit layer;

[0026] press-adhering said transfer sheet onto a surface of aninsulating sheet of a half-cured thermosetting resin in a manner thatthe wiring/circuit layer faces the surface of the insulating sheet; and

[0027] peeling off a resin film of the transfer sheet in order to obtaina circuit sheet having the wiring/circuit layer transferred onto thesurface of the insulating sheet in a manner of being buried therein.

[0028] According to the present invention, there is further provided amethod of producing a multi-layer wiring board by overlapping aplurality of the circuit sheets obtained above one upon another andheat-curing the insulating sheets while press-adhering them together.

[0029] The film with metal foil of the present invention has adistinguished feature in the use of the adhesive that is obtained bycrosslinking the acrylic polymer with a polyfunctional compound havingreactivity with the carboxyl group. That is, since the metal foil hasbeen stuck to the resin film with such an adhesive, the resin film canbe easily peeled off the metal foil. By utilizing this property, themetal foil can be transferred onto the surface of a soft resin sheet(e.g., a half-cured thermosetting resin sheet). Besides, the adhesiveexhibits excellent resistance against chemicals, i.e., exhibitsexcellent resistance even against the chemicals used for etching afterthe application of resist, exposure to light and developing, and againstthe chemicals used for the removal of resist. The adhesiveness is notdeteriorated even by such chemical solutions. Therefore, the film withmetal foil enables the metal foil to be easily formed into any circuitpatterns by applying the resist and etching.

[0030] As described above, the film with metal foil of the presentinvention enables the metal foil to be excellently transferred andformed in the shape of a circuit pattern. It is thus allowed to form atransfer sheet having a wiring/circuit layer on the surface thereof byusing the film with metal foil and, hence, to easily produce a circuitsheet for a multi-layer wiring substrate by using this transfer sheet.

[0031] According to the method of producing the circuit sheet for themulti-layer wiring board, the wiring/circuit layer formed of the metalfoil is transferred, in a buried form, onto the surface of thehalf-cured thermosetting resin sheet (corresponds to the core substrateof the multi-layer wiring board or to the insulating layer laminated onthe core substrate). Accordingly, the circuit sheet that is obtainedfeatures excellent surface smoothness, and makes it possible tofabricate a multi-layer wiring board having excellent surface smoothnessby heating and curing the thermosetting resin after having overlapped apredetermined number of pieces of the circuit sheets one upon the otherand having press-adhered them together.

[0032] In the multi-layer wiring board obtained by using the circuitsheet, no photosensitive resin has been used for forming the insulatinglayer. Even when the multi-layer wiring board is left to stand underhigh-temperature and high-humidity conditions, therefore, the insulatingproperty is not deteriorated that results from the hygroscopic property.

[0033] In such a multi-layer wiring board, further, the wiring/circuitlayer formed in each circuit sheet is buried in the insulating layerand, hence, the adhesion strength is high between the wiring/circuitlayer and the insulating layer. Even in the after-treatment such assolder reflow, therefore, the wiring/circuit layer is effectivelyprevented from being peeled off or swollen.

[0034] Further, the wiring/circuit layer is formed without platingcopper on the metal foil and, hence, does not become too thick lendingitself well for increasing the density of the wiring/circuit layer,decreasing the size of the board and decreasing the thickness.

[0035] When the multi-layer wiring board is to be produced by using theabove-mentioned film with metal foil, the insulating sheet constitutingthe insulating board and the wiring/circuit layer can be simultaneouslyformed through separate steps, enabling the productivity to be markedlyincreased.

DETAILED DESCRIPTION OF THE INVENTION Film with Metal Foil

[0036] The film with metal foil of the invention has a metal foil stuckto one surface of the resin film via an adhesive layer.

[0037] As the resin film, there can be used a known resin film having asuitable degree of flexibility such as of polyester like polyethyleneterephthalate, polyolefin like polypropylene or polyethylene, or a resinlike polyvinyl chloride, polyimide or polyphenylene sulfide.

[0038] It is desired that the resin film has a thickness of from 10 to500 μm and, particularly, from 20 to 300 μm. When the thickness issmaller than the above range, the wiring/circuit layer tends to bebroken due to deformation or folding of the film when the metal foil isworked into a circuit pattern to form the wiring/circuit layer. When thethickness is larger than the above range, on the other hand, the filmloses flexibility, and it becomes difficult to peel the film off themetal foil (wiring/circuit layer).

[0039] As the metal foil, there can be used the foil of a metal having asmall resistance suited for forming the wiring layer on the wiringboard, such as gold, silver, copper, aluminum or the like, or an alloycontaining at least one kind of such low-resistance metal.

[0040] It is desired that the metal foil has a thickness of from 1 to100 μm and, particularly, from 5 to 50 μm. When the thickness is smallerthan the above range, the wiring/circuit layer formed of the metal foilexhibits an increased resistivity and becomes not suited for producingthe wiring board. When the thickness is larger than the above range, onthe other hand, the insulating board or the insulating layer is greatlydeformed at the time of lamination for producing the wiring board thatwill be described later, the wiring/circuit layer is buried in anincreased amount when the wiring/circuit layer formed of the metal foilis transferred onto the insulating sheet, the insulating sheet isgreatly distorted, and deformation takes place when the resin is to becured. Besides, when the wiring/circuit layer is formed by etching themetal foil, it becomes difficult to effect the etching, making itdifficult to obtain a fine circuit maintaining high degree of precision.

[0041] In order to increase the adhering force to the resin film,further, the surface of the metal foil may be coarsened to form fineruggedness on the surface thereof on the side of the resin film. Forexample, fine ruggedness may be formed in the surface of the metal foilso as to possess the surface roughness Ra (JIS B0601) of from about 0.2to about 0.7 μm. In order to increase the adhering force between themetal foil and the resin film, further, a coupling agent may be appliedonto the surface of the metal foil. But, in order to easily peel off theresin film, it is desied that the coupling agent is not used.

[0042] The surface of the metal foil on the side opposite to the resinfilm may also be coarsened in the same manner as described above to formfine ruggedness of a similar surface roughness to enhance the adheringforce between the insulating sheet and the wiring/circuit layer.

[0043] As the adhesive for forming the adhesive layer, there can be usedthe one obtained by crosslinking an acrylic polymer which is obtained bythe copolymerization of a (meth)acrylic acid ester with a radicallypolymerizable monomer containing a carboxyl group, with a polyfunctionalcompound having a functional group reactive with the carboxylic acid.

[0044] The metal foil stuck onto the resin film using the above adhesivecan be formed into a circuit pattern by etching. Upon adhering thecircuit pattern onto the insulating sheet by the application ofpressure, further, the resin film can be peeled off without leaving theadhesive on the metal foil.

[0045] In the adhesive used in the present invention, there can befavorably used a (meth)acrylic acid ester represented by the followinggeneral formula (1),

CH₂═CR¹—COOR²  (1)

[0046] wherein R¹ is a hydrogen atom or a methyl group, and

[0047] R² is an alkyl group having 4 to 10 carbon atoms.

[0048] In the above general formula (1), the alkyl group of R² may be abutyl group, a pentyl group, a hexyl group, a heptyl group, an octylgroup, a nonyl group, a decyl group or a 4-methyl-1-pentyl group, or mayfurther be a substituted alkyl group in which some hydrogen atoms of thealkyl group are substituted by halogen atoms or hydroxyl groups.

[0049] As such a (meth)acrylic acid ester represented by the generalformula (1), there can be particularly preferably used a butyl(meth)acrylate or an octyl (meth)acrylate, which may be used in one kindor in a combination of two or more kinds in the invention.

[0050] The carboxyl group-containing radically polymerizable monomercopolymerized with the (meth)acrylic acid ester controls the polarity ofthe acrylic polymer that is formed, and works as crosslinking points byforming chemical bonds with the polyfunctional compound that will bedescribed later. Its examples include an acrylic acid and a methacrylicacid having a radically polymerizable unsaturated double bond and acarboxyl group in the molecules.

[0051] It is desired that the carboxyl group-containing radicallypolymerizable monomer is used in an amount of from 1 to 20 parts byweight and, particularly, from 3 to 10 parts by weight per 100 parts byweight of the (meth)acrylic acid ester. When the amount is smaller thanthe above range, it becomes difficult to obtain an adhesive having asuitable degree of adhesive force. When the amount is larger than theabove range, on the other hand, the glass transition point of the formedpolymer becomes too high or the crosslinking points are formed too much,making it difficult to obtain an adhesive having a suitable degree ofsoftness and adhesive force.

[0052] The polyfunctional compound for crosslinking the acrylic polymerthat is obtained by the copolymerization of the (meth)acrylic acid esterwith the carboxyl group-containing radically polymerizable monomer, has,in the molecules thereof, a plurality of functional groups capable ofreacting with the carboxyl groups, such as isocyanate groups, epoxygroups or metal chelate. Its concrete examples include tolylenediisocyanate (TDI), 4,4′-diphenylmethane diisocyanate (MDI), xylylenediisocyanate (XDI), aluminum trisethylacetoacetate (AlCH—YR), etc.

[0053] That is, such a polyfunctional compound reacts with the acrylicpolymer to form a crosslinking structure, making it possible to obtainan adhesive having a suitable degree of adhesiveness.

[0054] There can be further used a monofunctional compound having onefunctional group, such as butyl glycidyl ether together with the abovepolyfunctional compound. It is desired that the monofunctional compoundis used in an amount of not larger than 50 parts by weight per 100 partsby weight of the polyfunctional compound. That is, when the amount ofthe monofunctional compound becomes too great with respect to thepolyfunctional compound, it becomes difficult to obtain a sufficientdegree of crosslinking structure.

[0055] The mixture of the acrylic polymer and the polyfunctionalcompound can be easily crosslinked by applying the mixture to the metalfoil or the resin film followed by heating and drying.

[0056] It is desired that the degree of crosslinking is such that thegel fraction is not smaller than 70% by weight and, particularly, notsmaller than 80% by weight. The degree of crosslinking is related to thecohesive force of the adhesive at normal temperature or at a hightemperature. When the gel fraction is smaller than the above range, theadhesive strength becomes too great between the resin film and thewiring/circuit layer formed of the metal foil in the step of transfer,and it becomes difficult to peel off the resin film or the adhesiveremains on the wiring/circuit layer.

[0057] The gel fraction is calculated according to the following formula(2),

Gel fraction=(dry weight of filtrate after

immersed in the solvent)/(weight

of before immersed in the solvent)

×100  (2)

[0058] by using, as a solvent, an organic solvent such astetrahydrofuran (THF), immersing the adhesive in the solvent at atemperature of 30° C. a whole day with shaking so that it is swollenwith the solvent, filtering the undissolved components using a 200-meshgauze, drying the solvent, and measuring the weight thereof.

[0059] In the present invention, the adhesive must have a storagemodulus of shearing elasticity G′ of not smaller than 2×10⁵ dynes/cm²(2×10⁴ Pa) at 40° C. and a loss tangent tan δ of not smaller than 0.1 at40° C., and a storage modulus of shearing elasticity G′ of not smallerthan 5×10⁵ dynes/cm² (5×10⁴ Pa) at 120° C. and a loss tangent tan δ ofnot larger than 0.04, particulary not larger than 0.03 at 120° C.

[0060] When the storage modulus of shearing elasticity G′ at 40° C. issmaller than the above range, the adhesive becomes so soft that themetal foil is peeled off at the time of forming the wiring/circuit layerin the shape of a circuit pattern by etching the metal foil, and thewiring/circuit layer that is formed may be broken. When the loss tangenttan δ at 40° C. is smaller than the above range, too, the metal foil maybe peeled off during the etching and the wiring/circuit layer may bebroken.

[0061] Further, when the storage modulus of shearing elasticity G′ issmaller than the above range or when the loss tangent tan δ is largerthan the above range at 120° C., a problem may occur at the time ofpress-adhering and transferring the transfer sheet obtained by etchingthe metal foil onto the insulating sheet. That is, the transfer sheet ispress-adhered onto the insulating sheet to bury the wiring/circuitlayer, usually, at a temperature of from about 80 to about 150° C. Here,when the modulus of shearing storage elasticity G′ or the loss tangenttan δ at 120° C. lies outside the above-mentioned range, the adhesivefluidizes in the step of transfer and the wiring/circuit layer deformsmaking it difficult to form a fine wiring/circuit layer maintaining highprecision. Besides, the adhesive is adhered to the insulating sheet toan excess degree involving difficulty in peeling off the resin film.

[0062] In the present invention, the kinds of the acrylic polymer andpolyfunctional compound, amounts thereof as well as gel fraction thereofare adjusted, so that the storage modulus of shearing elasticity G′ andloss tangent tan δ at 40° C. and 120° C. lie within the above-mentionedranges, enabling the wiring/circuit layer to be formed by etching, andthe wiring/circuit layer to be transferred from the transfer sheethaving the wiring/circuit layer.

[0063] In the present invention, it is desired that the adhesive isblended with a plasticizer in an amount of from 0.05 to 5% by weight. Bybeing blended with the plasticizer, the adhesive force between the metalfoil and the adhesive can be elevated to a suitable degree. For example,the adhesive adheres to a sufficient degree to the surface of the metalfoil which is formed finely rugged and, hence, the etching solutioninfiltrates little into between the adhesive and the metal foil duringthe etching, effectively preventing the metal foil from being peeled offor the wiring/circuit layer from being broken. Further, since a suitabledegree of adhesive force is maintained, the resin film can be easilypeeled off even when the wiring/circuit layer formed by etching themetal foil is transferred onto the insulating sheet. It is consideredthat a trace amount of the plasticizer bleeds out to the surface of theadhesive layer to wet the surface of the metal foil. When the amount ofthe plasticizer is smaller than the above range, the above advantageresulting from the blending of the plasticizer is not exhibited to asufficient degree. When the plasticizer is blended in an amount largerthan the above range, on the other hand, the adhesive becomes so softthat the adhesive may remain on the wiring/circuit layer when, forexample, the wiring/circuit layer is transferred and the resin film ispeeled off.

[0064] As the plasticizer, there can be used an aliphatic dibasic acidsuch as adipic acid diester or the like, an aromatic dibasic acid estersuch as phthalic acid diester or the like, or a phosphorus acid-typeplasticizer. From the standpoint of compatibility with the acrylicpolymer and bleed-out property, however, it is desired to use a phthalicacid diester or an aliphatic dibasic acid ester. Among them, it is mostdesired to use the adipic acid diester and, particularly, adi-2-ethylhexyl adipate.

[0065] It is desired that the above adhesive is blended with an epoxyplasticizer in an amount of from 0.01 to 1% by weight. As the epoxyplasticizer, there can be exemplified an epoxylated soybean oil and anepoxylated linseed oil. By being blended with the epoxy plasticizer,peeling property is improved between the metal foil and the adhesive.When the epoxy plasticizer is blended in an amount of larger than 1% byweight, the adhesive force is not obtained to a sufficient degreebetween the metal foil and the resin film, permitting the peeling tooccur in the step of etching.

[0066] The adhesive may be further blended with a benzotriazole-typeanti-corrosive agent. By being blended with the anti-corrosive agent,the metal foil is effectively prevented from being excessively corrodedat the time of etching and, particularly, fine wiring portions andcorner portions of the wiring/circuit layer are effectively preventedfrom being excessively corroded. The anti-corrosive agent can be blendedin any amount as far as the adhesive does not lose the above-mentionedproperties.

[0067] The above-mentioned plasticizer and the anti-corrosive agent canbe blended at any stage. Generally, however, the acrylic polymer ofbefore being crosslinked is blended with the plasticizer and theanti-corrosive agent.

[0068] The film with metal foil of the invention can be obtained byapplying the above adhesive to the surface of the metal foil or to thesurface of the resin film, and press-adhering them together. In thiscase, it is desired that the adhesive layer formed of the adhesive has athickness of from about 1 to about 30 μm. When the thickness is toosmall, the metal foil is not held on the resin film with a sufficientlylarge adhesive force, and the metal foil tends to be peeled off. Whenthe thickness is too large, on the other hand, the adhesive tends tostay on the wiring/circuit layer when the resin film is peeled off afterthe wiring/circuit layer is transferred.

[0069] The thus obtained film with metal foil of the invention exhibitsan adhesive force of the adhesive layer of, usually, from 50 to 700 g/20mm (about 0.5 to 7.0 N/20 mm). Therefore, the metal foil is held withoutbeing peeled off even at the time of etching, and the resin film can beeasily peeled off after the wiring/circuit layer is transferred. Theadhesive force of the adhesive layer is measured in compliance with 180°peeling strength (JIS-Z-0237) at the time of peeling the resin film off.

Production of a Circuit Sheet for the Multi-layer Wiring Board

[0070] According to the present invention, the metal foil of theabove-mentioned film with metal foil is shaped into a circuit pattern toform the wiring/circuit layer in order to obtain a transfer sheet. Thetransfer sheet is then press-adhered onto an insulating sheet totransfer the wiring/circuit layer onto the surface of the insulatingsheet. Then, the resin film of the transfer sheet is peeled off toproduce a circuit sheet for the multi-layer wiring board.

[0071] Preparation of the Transfer Sheet:

[0072] In order to form the metal foil of the film into the circuitpattern, there can be employed a resist method known per se.

[0073] That is, a photoresist is applied onto the whole surface of themetal foil, which is then exposed to light through a mask of apredetermined pattern. After developing, the metal foil is removed fromthe non-pattern portions (potions from where the photoresist has beenremoved) by etching such as plasma etching or chemical etching. Thus,there is formed a wiring/circuit layer on which the metal foil is formedin the shape of a circuit pattern. It is, of course, allowable to applythe photoresist onto the surface of the metal foil in the shape of apredetermined circuit pattern by screen-printing or the like and, then,to form the wiring/circuit layer by etching after exposure to light inthe same manner as described above.

[0074] After the etching, the resist may remain on the wiring/circuitlayer. By removing the remaining resist with a resist-removing solutionfollowed by washing, it is allowed to obtain a transfer sheet having thewiring/circuit layer formed on the surface of the resin film.

[0075] In the present invention, the transfer sheet having thewiring/circuit layer is prepared as described above by using theabove-mentioned film with metal foil. The metal foil is held by theresin film with a suitable degree of adhesive force. Besides, theadhesive layer exhibits resistance against chemicals used at the time ofetching and suppresses the infiltration of the chemical solution,effectively preventing such an inconvenience that the metal foil ispeeled off when the wiring/circuit layer is being formed by etching. Asa result, it is allowed to form a fine and highly dense wiring/circuitlayer maintaining a high precision.

[0076] Insulating Sheet:

[0077] The insulating sheet onto which will be transferred thewiring/circuit layer formed on the transfer sheet comprises a half-curedthermosetting resin. As the thermosetting resin, there can be usedpolyphenylene ether (PPE), a bismaleimide resin such as BT (bismaleimidetriazine) resin or the like, or a maleimide resin, an epoxy resin, apolyimide resin, a fluorine-contained resin or a phenolic resin.Preferably, there is used a thermosetting resin which is liquid at roomtemperature.

[0078] The insulating sheet corresponds to the insulating substrate ofthe circuit board. In order to maintain a strength, in general, thefiller is used together with the thermosetting resin. As the filler,there can be used an organic or inorganic powder or fiber.

[0079] As the inorganic filler, there can be used SiO₂, Al₂O₃, ZrO₂,TiO₂, AlN, SiC, BaTiO₃, SrTiO₃, zeolite, CaTiO₃ and aluminum borate thathave been known per se. It is desired that these inorganic fillers arein the form of a nearly spherical powder having an average particlediameter of not larger than 20 μm, preferably, not larger than 10 μmand, most preferably, not larger than 7 μm, but may be in the form of afiber having an average aspect ratio of not smaller than 2 and,particularly, not smaller than 5. Examples of the fiber filler include aglass, a woven fabric or a nonwoven fabric having any property. As theorganic filler, there can be exemplified an aramid fiber, a cellulosefiber, etc.

[0080] The above various fillers can be used alone or in a combinationof two or more kinds. In general, it is desired that the thermosettingresin and the filler are used at a ratio of thermosettingresin/filler=15/85 to 65/35 on the volume basis.

[0081] The insulating sheet is obtained by forming a slurry thatcontains the thermosetting resin or that contains the thermosettingresin and the filler into a sheet by the doctor blade method, andheating it to a semi-cured state.

[0082] It is further desired to form via-holes in the half-curedinsulating sheet by the carbonic acid gas laser method or the likemethod, and fill the via-holes with a powder of a low-resistance metalsuch as gold, silver, copper or aluminum to form via-hole conductors.

[0083] The thickness of the insulating sheet is suitably selecteddepending upon the thickness of the multi-layer wiring board that is tobe finally obtained.

[0084] Transfer:

[0085] In the present invention, the transfer sheet described above isoverlapped on the insulating sheet in a manner that the wiring/circuitlayer faces the insulating sheet and is press-adhered thereto. In thiscase, the portion where the surfaces of the via-hole conductors formedin the insulating sheet are exposed, is overlapped on the wiring/circuitlayer.

[0086] The press-adhesion is effected with a pressure of about 0.01 toabout 0.5 N/cm², so that the wiring/circuit layer is completely buriedin the surface of the insulating sheet. It is desired that thepress-adhesion is usually conducted being heated at about 80 to 150° C.Therefore, the wiring/circuit layer that is buried is firmly held by theinsulating sheet. By coarsening the surface of the metal foil, further,fine ruggedness formed in the surface of the wiring/circuit layer mesheswith the surface of the insulating sheet, and the junction strength isfurther enhanced between the wiring/circuit layer and the insulatingsheet.

[0087] Then, by peeling the resin film off the transfer sheet, thewiring/circuit layer is transferred onto the surface of the insulatingsheet, and there is obtained a circuit sheet having the wiring/circuitlayer on the surface.

[0088] The thus obtained circuit sheet has the wiring/circuit layerburied in the surface of the sheet, and has a very high degree offlatness making it possible to obtain a multi-layer circuit board havingexcellent flatness suited for mounting even a flip chip.

[0089] (Production of the Multi-layer Wiring Board)

[0090] The wiring board of a single layer is obtained by completelycuring the insulating sheet by heating the above circuit sheet.Manufacturing the multi-layer wiring boards by using the circuit sheetis advantageous for enhancing the productivity of the multi-layer wiringboards.

[0091] That is, a predetermined number of circuit sheets (e.g., circuitsheets in which via-holes are formed) obtained above are overlapped oneupon the other in a manner that the wiring/circuit layers and theinsulating sheets are alternatingly positioned, and are press-adheredtogether followed by heating to completely cure the insulating sheets atthe same time, thereby to obtain a desired multi-layer wiring board.

[0092] According to this method, preparation of the insulating sheetequipped with via-hole conductors for constituting the insulatingsubstrate and the preparation of the transfer sheet equipped with thewiring/circuit layer are simultaneously conducted through separate stepsto accomplish a very high production efficiency.

[0093] In the obtained multi-layer wiring board, the wiring/circuitlayer formed in each layer is completely buried in the insulatingsubstrate of each layer, and the wiring/circuit layer of the uppermostlayer, too, is buried in the surface. Accordingly, the multi-layerwiring board has very excellent flatness so as to be adapted to mountingeven a flip chip, and its thickness can be decreased as much aspossible.

[0094] Further, the multi-layer wiring board does not have to use thephotosensitive resin as the insulating substrate material. Even when themulti-layer wiring board is left to stand under high-temperature andhigh-humidity conditions for extended periods of time, therefore,reliability of the circuit is not impaired. Moreover, the wiring/circuitlayer is directly formed from the metal foil without the need ofeffecting the plating on the metal foil. Therefore, the wiring/circuitlayer is not formed too much thickly, making it possible to effectivelyprevent the deformation of the board and to prevent a decrease in thestrength thereof caused by an increase in the thickness of thewiring/circuit layer, which is very advantageous for realizing the boardin a small size.

EXAMPLES Example 1

[0095] Via-holes of,a diameter of 0.1 mm were formed in a prepreg (150μm thick) of an arylated polyphenylene ether (A-PPE) by the carbonicacid gas laser beam and were filled with a copper paste containing asilver-plated copper powder to form via-hole conductors and to preparean insulating sheet for a multi-layer wiring board.

[0096] An acrylic polymer obtained by the copolymerization of 100 partsby weight of a butyl acrylate with 10 parts by weight of an acrylicacid, was mixed with a polyfunctional compound (an adduct of TDI(tolylene diisocyanate) of trimethylolpropane, Coronate L-45manufactured by Nihon Polyurethane Co.), a plasticizer (d)(di-2-ethylhexyl adipate) and an anti-corrosive agent (benzotriazole),and the mixture was melt-kneaded at ratios shown in Table 1 to obtain anadhesive having a gel fraction and an index of viscoelasticity as shownin Table 1. The index of viscoelasticity was measured based on dynamicviscoelasticity spectra (frequency of 10 Hz, temperature rising rate of2° C./min).

[0097] The adhesive was applied onto the surface of a 25 μm-thickpolyethylene terephthalate (PET) film to prepare an adhesive film onwhich the adhesive is applied maintaining a thickness of 20 μm.

[0098] Further, an electrolytic copper foil (12 μm thick) formed by theelectrolytic plating method and having a surface roughness (Ra) of 0.5μm was adhered onto the adhesive surface of the adhesive film to preparea film with metal foil. The surface of the copper of the film wascoarsened by etching with a formic acid to adjust the surface roughness(Ra) to be 0.5 μm.

[0099] The film with metal foil was measured for its peeling force at40° C., peeling force at 120° C., and re-peeling force at 40° C. at apeeling rate of 300 mm/min by the methods described below. The resultswere as shown in Table 3.

[0100] Peeling Force at 40° C.:

[0101] The film immediately after the metal foil was adhered thereon wasleft to stand at 40° C. for 30 minutes, and was measured for its 180°peeling strength at 40° C.

[0102] Peeling Force at 120° C.:

[0103] The film immediately after the metal foil was adhered thereon wasleft to stand at 120° C. for 30 minutes, and was measured for its 180°peeling strength at 120° C.

[0104] Re-peeling Force at 40° C.:

[0105] The pressure of 30 kg/cm² (about 300 N/cm²) was applied at 130°C. for 30 seconds, and the peeling force was measured at 40° C.

[0106] A photosensitive resist was applied onto the surface (coarsenedsurface) of the copper foil of the film, a circuit pattern was formed byexposure to light through a glass mask, and the film was immersed in asolution of ferric chloride to form a wiring/circuit layer of the shapeof the circuit pattern by removing the non-patterned portions by etchingat an etching rate of 35 μm/minute. Further, the resist remaining on thewiring/circuit layer was removed by using an aqueous solution of sodiumhydroxide in order to obtain a transfer sheet having the wiring/circuitlayer formed on the PET film via the adhesive layer.

[0107] The circuit pattern on the wiring/circuit layer possessed linesof a width of 1 mm, and fine lines of a width of 50 μm maintaining a gapof 50 μm, and possessed an area corresponding to 50% that of theinsulating sheet prepared above.

[0108] In effecting the etching, infiltration of the solution intobetween the adhesive layer and the metal foil was observed. The resultswere as shown in Table 3. In the Table, ◯ represents a case of when noinfiltration of solution was found, and X represents a case of when theinfiltration was found.

[0109] The thus prepared transfer sheet was overlapped on the insulatingsheet being so positioned that the wiring/circuit layer faced thevia-hole conductors, and was pressed with a pressure of 30 kg/cm² (about300 N/cm²) at 130° C. for 30 seconds so that the wiring/circuit layer ofthe transfer sheet was buried in the insulating sheet. Then, the PETfilm of the transfer sheet was peeled off together with the adhesivelayer to obtain a circuit sheet on which the wiring/circuit layer hadbeen transferred.

[0110] It was confirmed that in this circuit sheet, the wiring/circuitlayer had been completely buried in the surface of the insulating sheet,and the surface of the insulating sheet and the surface of thewiring/circuit layer had been positioned on the same plane.

[0111] In peeling off the PET film, presence of the adhesive wasobserved on the upper surface of the transferred wiring/circuit layer,and the results were listed on the column of transfer peeling of Table4. In this Table, ◯ represents a case of when there remained noadhesive, and X represents a case when there maintained the adhesive.

[0112] Six pieces of the circuit sheets were prepared in a manner asdescribed above, overlapped one upon the other in a manner that thewiring/circuit layers and the insulating sheets were alternatinglyarranged, and were press-adhered together with the application ofpressure of 30 kg/cm² (about 300 N/cm²) at 200° C. for one hour toobtain a multi-layer wiring board of a six-layer structure.

[0113] The obtained multi-layer wiring board was subjected to thesoldering test and bias test in accordance with the following methods,and the results were as shown in Table 4.

[0114] Soldering Test:

[0115] After heated for soldering at 260° C. for 2 minutes, defectivelamination was observed between the wiring/circuit layer and theinsulating layer. Twenty pieces of the multi-layer wiring boards weresubjected to the same testing, and the resistance against the solderingheat was evaluated in terms of the frequency of occurrence of defect inthe lamination.

[0116] Bias Test:

[0117] A sample board was left to stand in an atmosphere of atemperature of 130° C. and a relative humidity of 85% for 300 hours.Then, a voltage of 5.5 V was applied to measure the insulatingresistance of fine wire portions in the wiring/circuit layer.

[0118] Twenty pieces of the sample boards were subjected to the sametesting. The sample boards exhibiting the insulating resistance of notlower than 10⁹Ω were regarded to be of good quality, and thehigh-temperature high-humidity properties were evaluated in terms of thenumber of boards exhibiting insulating resistances smaller than theabove value.

Examples 2 to 4, Comparative Examples 1 to 4

[0119] Films with metal foil, transfer sheets, circuit sheets andmulti-layer wiring boards were prepared in quite the same manner as inExample 1 and were observed and evaluated in the same manner as inExample 1, but using adhesives prepared by changing the amount ofacrylic acid in the acrylic polymer, and blending the polyfunctionalcompound, plasticizer (α) (di-2-ethylhexyl adipate) and anti-corrosiveagent in amounts as shown in Tables 1 and 2, and were observed andevaluated in the same manner as in Example 1. The results were as shownin Tables 3 and 4.

Example 5

[0120] A film with a metal foil, a transfer sheet, a circuit sheet and amulti-layer wiring board were prepared in quite the same manner as inExample 1 but using an adhesive prepared by using a monofunctionalcompound (butyl glycidyl ether, BGE manufactured by Nihon Kayaku Co.) inaddition to a polyfunctional compound as shown in Table 1, and wereobserved and evaluated in the same manner as in Example 1. The resultswere as shown in Tables 3 and 4.

Example 6 and 7, Comparative Examples 5 and 6

[0121] Films with metal foil, transfer sheets, circuit sheets andmulti-layer wiring boards were prepared in quite the same manner as inExample 2, but using adhesives prepared by using a plasticizer (β)(epoxylated soybean oil manufactured by Asahi Denka Co., trade name:adekacizer-O-130P) in addition to the plasticizer (α) (di-2-ethylhexyladipate) and changing the amount of the epoxy plasticizer anddi-2-ethylhexyl adipate as shown in Tables 1 and 2, and were observedand evaluated in the same manner as in Example 1. The results were asshown in Tables 3 and 4. TABLE 1 Example 1 2 3 4 5 6 7 Butylacrylate/parts by weight 100 100 100 100 100 100 100 Acrylic acid/partsby weight  10  10  10  4  10  10  10 Polyfunctional compound/  3  4  7 3 poly: 3  4  4 parts by weight mono: 1 Plasticizer (α)/  2  2  2  2  2 2  4 parts by weight Anti-corrosive/parts by weight  3  3  3  3  3  3 3 Plasticizer (β)/  0  0  0  0  0    0.2    0.2 parts by weight Gelfraction/% by weight  85  90  97  95  90  87  83 Index ofviscoelasticity 40° C.; G′ (dyne/cm²) 6E+6 6E+5 1E+6 2E+5 6E+5 6E+5 6E+5tan δ 2E−1 2E−1 4E−1 3E−1 2E−1 2E−1 1E−1 120° C.; G′ (dyne/cm²) 5E+56E+5 7E+5 6E+5 6E+5 5E+5 5E+5 tan δ 2E−3 3E−3 2E−3 2E−3 3E−3 1E−2 1E−2

[0122] TABLE 2 Comparative Example 1 2 3 4 5 6 Butyl acrylate/parts by100 100 100 100 100 100 weight Acrylic acid/parts by  10  1  1  15  10 10 weight Polyfunctional compound/    2.5  4  7  4  3  4 parts byweight Plasticizer (α)/  2  2  2  2  4  4 parts by weightAnti-corrosive/parts by  3  3  3  3  3  3 weight Plasticizer (β)/  0  0 0  0  2  0 parts by weight Gel fraction/% by weight  60  80  90  60  80 83 Index of viscoelasticity 40° C.; G′ (dyne/cm²) 3E+5 1E+5 2E+5 3E+54E+5 5E+5 tan δ 1E−1 2E−1 1E−1 5E−1 4E−1 7E−1 120° C.; G′ (dyne/cm²)3E+5 1E+5 2E+5 1E+5 4E+5 4E+5 tan δ 2E−2 5E−2 3E−2 5E−2 1E−2 2E−2

[0123] In Tables 1 and 2, the plasticizer (α) was the di-2-ethylhexyladipate, and the plasticizer (β) was the epoxylated soybean oil.

[0124] The amounts of the polyfunctional compounds, plasticizers (α) and(β), and anti-corrosive agents are per 100 parts by weight of theacrylic polymer.

[0125] In the column of polyfunctional compound of Example 5 in Table 1,“poly” represents the blending amount of the polyfunctional compound and“mono” represents the blending amount of the monofunctional compound.TABLE 3 Peeling force Peeling force Re-peeling force at 40° C. (N/cm) at120° C. (N/cm) at 40° C. (N/cm) Etching Example 1 5.59 0.98 9.11 ◯Example 2 3.92 0.98 6.62 ◯ Example 3 2.45 0.49 4.29 ◯ Example 4 1.960.29 3.14 ◯ Example 5 4.30 0.99 7.71 ◯ Example 6 4.12 0.88 5.59 ◯Example 7 6.27 1.32 6.76 ◯ Comp. Example 1 7.84 1.47 12.94 ◯ Comp.Example 2 2.45 0.10 4.04 X Comp. Example 3 1.47 0 2.50 ◯ Comp. Example 44.41 0.10 7.94 ◯ Comp. Example 5 2.40 0 2.00 ◯ Comp. Example 6 6.00 1.1510.55 ◯

[0126] TABLE 4 Transfer Solding Bias peeling test test Example 1 ◯ 0/200/20 Example 2 ◯ 0/20 0/20 Example 3 ◯ 0/20 0/20 Example 4 ◯ 0/20 0/20Example 5 ◯ 0/20 0/20 Example 6 ◯ 0/20 0/20 Example 7 ◯ 0/20 0/20 Comp.Example 1 X 19/20  0/20 Comp. Example 2 ◯ 0/20 20/20  Comp. Example 3 X20/20  0/20 Comp. Example 4 X 20/20  0/20 Comp. Example 5 X 20/20  0/20Comp. Example 6 X 20/20  0/20

[0127] When a multi-layer wiring board is produced by using the filmwith metal foil of the present invention as will be comprehended fromthe results of above Examples, the metal foil is held with an adhesivemaintaining a suitable adhering force. Therefore, even when thewiring/circuit layer is formed from the metal foil by etching, the metalfoil is not peeled off or the wiring/circuit layer is not broken, andthe etching solution does not infiltrate into between the adhesive andthe metal foil. Further, the resin film can be easily peeled off in astate where the wiring/circuit layer is buried in the half-curedinsulating sheet. At this moment, the adhesive is all peeled offtogether with the resin film without remaining on the wiring/circuitlayer that is transferred.

[0128] Since the wiring/circuit layer is buried in the insulatingsubstrate, the obtained multi-layer wiring board has a favorably flatsurface enabling even a flip chip to be mounted thereon. Besides, a dropin the strength of the wiring board is effectively prevented thatresults from defective lamination of the layers.

[0129] The present invention is very advantageous for the production ofthe multi-layer wiring boards which are small in size, small inthickness, and possess highly dense fine circuitry maintaining a highprecision. Besides, the production efficiency is markedly improved sincethe formation of the insulating sheet and the formation of thewiring/circuit layer can be simultaneously conducted through separatesteps.

1. A film with metal foil in which a metal foil is stuck to one surfaceof a resin film via an adhesive layer; wherein said adhesive layer isformed of an adhesive obtained by crosslinking an acrylic polymerobtained by the copolymerization of a (meth)acrylic acid ester with acarboxyl group-containing radically polymerizable monomer, with apolyfunctional compound having a functional group reactive with thecarboxyl group; and said adhesive has a storage modulus of shearingelasticity G′ of not smaller than 2×10⁵ dynes/cm² and a loss tangent tanδ of not smaller than 0.1 at 40° C., and a storage modulus of shearingelasticity G′ of not smaller than 5×10⁵ dynes/cm² and a loss tangent tanδ of not larger than 0.01 at 120° C.
 2. The film with metal foilaccording to claim 1, wherein said acrylic polymer is a copolymer of 100parts by weight of a (meth)acrylic acid ester and 1 to 20 parts byweight of a carboxyl group-containing radically polymerizable monomer.3. The film with metal foil according to claim 2, wherein said(meth)acrylic acid ester is represented by the following general formula(1), CH₂═CR¹—COOR²  (1)wherein R¹ is a hydrogen atom or a methyl group,and R² is an alkyl group having 4 to 10 carbon atoms.
 4. The film withmetal foil according to claim 1, wherein said adhesive is obtained bycrosslinking the acrylic polymer with said polyfunctional compound inthe presence of a monofunctional compound having said functional group.5. The film with metal foil according to claim 1, wherein said adhesiveis so crosslinked that the gel fraction which represents the amount ofthe adhesive that does not dissolve in tetrahydrofuran is not smallerthan 70% by weight.
 6. The film with metal foil according to claim 1,wherein said adhesive contains a plasticizer in an amount of from 0.05to 5% by weight.
 7. The film with metal foil according to claim 6,wherein said plasticizer is a phthalic acid diester or an aliphaticdibasic acid ester.
 8. The film with metal foil according to claim 7,wherein said adhesive further contains an epoxy plasticizer in an amountof from 0.01 to 1% by weight.
 9. The method of producing a circuit sheetfor a multi-layer wiring board comprising: working a metal foil of afilm with metal foil into a circuit pattern by a resist method therebyto prepare a transfer sheet having a wiring/circuit layer;press-adhering said transfer sheet onto the surface of an insulatingsheet of a half-cured thermosetting resin in a manner that thewiring/circuit layer faces the surface of the insulating sheet; andpeeling off the resin film of the transfer sheet in order to obtain acircuit sheet having the wiring/circuit layer transferred onto thesurface of the insulating sheet in a manner of being buried therein. 10.The method of producing a circuit sheet for a multi-layer wiring boardaccording to claim 9, wherein via-hole conductors are formed in saidinsulating sheet prior to transferring said wiring/circuit layer ontosaid insulating sheet.
 11. A method of producing a multi-layer wiringboard by overlapping a plurality of the circuit sheets obtained by themethod of production of claim 9 one upon another and heat-curing theinsulating sheets while press-adhering them together.